Speaking to a forum at Georgetown University here in Washington, Rasmussen said he telephoned Afghan President Hamid Karzai to apologize for the air strike in southern Afghanistan.

"Of course, there will be bad days when we cause unintended civilian casualties," said Anders Fogh Rasmussen. "I just spoke to President Karzai and expressed my deep regrets and condolences for the latest incidents where Afghan civilians have lost their lives."

NATO says its planes fired on what was thought to be a group of insurgents in Uruzgan province.

The commander of NATO forces in Afghanistan, U.S. Army General Stanley McChrystal, expressed regret for the civilian deaths and pledged to cooperate with a joint investigation.

In recent months, NATO has limited air strikes and tightened rules of engagement on the battlefield to protect Afghan civilians and win their loyalty from the Taliban.

During his remarks, the NATO secretary-general said such deaths have declined in recent months.

Rasmussen said that based on statistics compiled by the United Nations, the Taliban are responsible for most civilian deaths in Afghanistan.

"A huge majority of civilian casualties are caused by the Taliban and we also know that the Taliban use, or misuse, women and children - innocent civilians - as a human shield on roofs, in windows, and otherwise," he said. "So they do not care."

Uruzgan is northeast of Helmand province, where NATO and Afghan forces are engaged in a major offensive against Taliban insurgents in Marja.

As soon as the area is secure, NATO says it plans to rush in a civilian administration, restore public services and pour in aid.

The effort is seen as a key test of U.S. President Barack Obama's strategy for reversing the rise of the Taliban, while protecting the civilian population.

NATO's Anders Fogh Rasmussen says he believes the new plan is already having a positive impact.

"I believe that these new forces and our new approach are already having an effect on the ground," said Rasmussen. "There is new momentum. I am confident that this year we will be able to start transferring security responsibilities to the Afghans themselves."

Rasmussen says the main lesson learned so far from the war in Afghanistan is that there is no military solution to such conflicts.

He says providing good governance, civilian reconstruction and humanitarian assistance are the keys to long-term peace and stability in Afghanistan.